1. Field of the Invention
The present invention relates to billets by continuous casting, in particular relates to a high carbon steel billet by continuous casting and a manufacturing method therefor by continuous casting, and more specifically it relates to a billet by continuous casting having a small amount of central segregation in its center and a manufacturing method therefor.
2. Description of the Related Art
When bar steel, represented typically by a rod and a bar, is manufactured, a billet in a shape of a square column having a length of one side of no more than 200 mm or a cylindrical column having a diameter of no more than 200 mm is manufactured which in turn is rolled to produce various steel for a bar. When the billet is conventionally manufactured, a bloom having a large cross-section is produced by continuous casting so as to produce the billet by blooming mill. However, it is preferable for simplification of the manufacturing process and promotion of energy saving to produce the billet directly by continuous casting. Therefore, the continuous casting of billets has been carried out mainly for low carbon and medium carbon steel having carbon contents of 0.05 to 0.3% by mass.
The continuous casting of steel involves a problem that impurities in the steel are condensed to be concentrated in the central portion of a cast slab to produce central segregation. When the concentration of the segregation component is large or the range of the central segregation portion is large, in the manufacturing of rod, for example, breaking of wire occurs during wire drawing for producing wire because hardness in the central segregation portion is different from those in other portions. In the case of a cast slab, in the manufacturing of thick plates, for example, a problem that toughness of the central segregation portion in the produced thick-plate is reduced and so forth arises.
The problem of the central segregation arises in producing billets directly by continuous casting just like in slab and bloom. When the carbon content in steel is high, the central segregation has a profound effect on billets. When the high carbon steel billet, as a material, is rolled for producing rod, the central segregation portion of the billet grows to be pro-eutectoid cementite and micro-martensite after rolling of rod, so that cracks originated from the pro-eutectoid cementite and micro-martensite are produced in the rod during wire drawing, resulting in breaking of wire in the rod.
A technique for reducing central segregation in the continuous casting in slab and bloom is known in which an equiaxed crystal rate in the central portion of a cast slab or bloom is increased by reducing the degree of super heat of liquid steel to be poured in a mold. In a billet by continuous casting, reducing the degree of super heat of liquid steel in a mold can also reduce the central segregation thereof. However, the cross-sectional size of a mold in continuous billet casting is small and the internal diameter of a pouring nozzle is also small. Accordingly, when liquid steel having a low degree of super heat is cast, the liquid steel coagulates in the pouring nozzle, so that the nozzle is plugged so as to be susceptible to a trouble of shutting down of casting. Therefore, in continuous billet casting, reducing the degree of super heat of liquid steel is difficult to be adopted as means for reducing the central segregation.
In a slab and a bloom caster, a technique for reducing central segregation is also known in which mechanical soft reduction is carried out with rolls on a cast slab or bloom so as to prevent the liquid steel in the central portion from fluidization by coagulation and contraction to thereby improve the central segregation. When the mechanical soft reduction technique is tried to apply it as it is to the billet, approximate twenty rolls for the mechanical soft reduction are needed to be arranged in the range of approximate 10-m length just like in the slab and the bloom caster. The billet continuous caster has a feature that the number of pinch rolls per one strand is about 5 pairs; however the simplicity in equipment of the billet continuous caster will be lost when a number of the mechanical soft reduction rolls are arranged just like in the slab and the bloom caster.